Merges fund-index pairs into the fund-index map (.fundsr$fund_index_map).
Existing entries with the same names are replaced.
See also
Other fund-index map mutators:
clear_fund_index_map(),
get_fund_index_map()
Examples
add_fund_index_map(c(fund1 = "INDEX1", fund2 = "INDEX2", fund3 = "INDEX2"))
